The present invention generally relates to sub-micron CMOS serial bus transceiver chips that are supplied power by means of a voltage regulator on a printed-circuit board (PCB).
The transceiver may be required to withstand a fault condition whereby its output is accidentally shorted to the 5 v power supplied by a bus cable. The transceiver and all other circuitry (related and unrelated) on the chip must not be damaged during the fault condition.
When the driver is back-driven by the cable, there are two low-impedance feedthrough paths available for the cable to charge-up the PCB power supply. One is through the pfet (p-channel field effect transistor) transistor channel, and the other is through the pfet drain-well diode, which may become forward biased. Since the power-supply regulator is typically not designed to sink current, it will present a high DC impedance to the cable power, and eventually charge up to the cable voltage. Therefore, all devices on the chip with the transceiver will potentially be exposed to damaging overvoltage conditions. The p-channel fets in present sub-micron CMOS processes are especially susceptible to long-term degradation from elevated power-supply voltages.
Existing approaches to protecting devices form voltage feedthrough onto the power supply employ comparison circuits that sense the voltage on the transceiver output rather than its power supply. These comparison circuits control the well and gate of the driver pfet transistor to shut off the feedthrough paths by means of additional fet (field effect transistor) switches. The comparison circuitry and gate/well control switches may themselves create new sensitivities to overvoltages, therefore necessitating their own protection circuitry. The final circuit implementations can often be complicated and time-consuming to verify. Another drawback of this approach is that it eliminates the clamping action of the driver pfet, often necessitating use of additional, costly processing step to implement a diode.
A general object of an embodiment of the present invention is to provide a power supply feedthrough protection circuit which effectively solves the problem of voltage feedthrough from the bus cable charging the chip power-supply beyond the acceptable transistor limits.
Another object of an embodiment of the present invention is to provide a power supply feedthrough protection circuit which continuously senses the chip power-supply, compares the supply to an acceptable threshold, and provides a low-impedance current path as soon as the power-supply exceeds the threshold.
Still another object of an embodiment of the present invention is to provide a power supply feedthrough protection circuit which allows a driver to draw current from a cable, in a controlled manner.
Still yet another object of an embodiment of the present invention is to provide a power supply feedthrough protection circuit which provides that a power supply is never allowed to exceed a maximum allowable limit for transistors.
Briefly, and in accordance with at least one of the foregoing objects, an embodiment of the present invention provides a power supply feedthrough protection circuit which includes means for continuously sensing a chip power supply, means for dividing the chip power supply to a reduced voltage to be compared to the reference voltage, means for comparing the reduced voltage to the reference voltage to determine whether the chip power supply exceeds an acceptable threshold, and means for providing a low-impedance current path when it is determined that the chip power supply exceeds the threshold.